A Wi-Fi network may be formed by one or more access points (APs) that provide a wireless communication channel or link with a number of client devices or stations (STAs). Each AP, which may correspond to a Basic Service Set (BSS), periodically broadcasts beacon frames to enable any STAs within wireless range of the AP to establish and/or maintain a communication link with the Wi-Fi network. The beacon frames, which may include a traffic indication map (TIM) indicating whether the AP has queued downlink data for the STAs, are typically broadcast according to a target beacon transmission time (TBTT) schedule.
An independent Basic Service Set (IBSS) network (also known as an ad-hoc wireless network or a peer-to-peer 802.11 wireless network) may be formed by a number of STAs without the presence of an AP. For an IBSS network, each STA belonging to the IBSS network may attempt to broadcast a beacon frame at each of the TBTTs. Because there is not an AP in an IBSS network, the beacon frame transmissions are distributed between the STAs in the IBSS network. Thus, at each TBTT, each STA associated with the IBSS network uses a backoff timer (e.g., a random number between 0 and twice the minimum contention window) to contend for medium access. The STA having the shortest backoff period wins the contention period for medium access, and may broadcast its beacon frame for the current TBTT interval. All other STAs receive the broadcasted beacon frame and terminate their contention for medium access for the current TBTT interval.
The broadcasted beacon frame includes the transmitting STA's timing synchronization function (TSF) value. The other STAs may synchronize their own local TSF values with the broadcasted TSF value if the broadcasted TSF value is greater than the local TSF value so that all the STAs are synchronized with each other. Timing synchronization is important, for example, so that all the STAs may wake-up and contend for medium access at the correct same times.
As mentioned above, STAs typically use a backoff timer when contending for medium access (e.g., to broadcast a beacon frame). The backoff timer values are random, and therefore may reduce the probability of beacon frame collisions resulting from multiple STAs attempting to broadcast beacon frames at the same time. However, because synchronization of TSF values between the STAs in an IBSS network is relatively slow (e.g., compared with a BSS network), and because there is not a global or “master” TSF value in the IBSS network (e.g., compared with the global TSF value maintained by an AP in a BSS network), it may be difficult to maintain synchronization of all the STAs' TSF values in the IBSS network. Discrepancies between the STAs' TSF values may lead to beacon frame collisions and/or to other timing related problems. These problems may be exacerbated when one of the STAs acts as a forwarding device in a multi-hop IBSS network, for example, because of difficulties in synchronizing multiple IBSS sub-networks across a forwarding device.
Thus, there is a need to improve timing synchronization between the STAs in an IBSS network, for example, to reduce the probability of beacon frame collisions.